The present invention relates generally to data communication, and more specifically to a satellite packet communication system.
In terrestrial data communication systems, a special protocol, such as basic mode control procedure, is employed at opposite ends of a link to ensure a high degree of reliability of transmission. The basics of the protocol require the receiving end of the link to return an acknowledgement whenever it receives a message and require the transmitting end to await the receipt of the acknowledgement prior to the transmission of a subsequent message. If the acknowledgement is not received, the receiving end requests the transmitting end to retransmit a copy of the previous data unit. If a satellite channel is used in a data communication system, however, the inherent up-and-down propagation delay of the satellite channel would severely reduce the throughput of the system. One approach would be to interpose a satellite delay compensation unit (SDCU) between a data terminal equipment and one end of a satellite channel. The satellite delay compensation unit returns an acknowledgement to the data terminal equipment whenever it receives a message according to a conventional protocol and allows consecutive transmission of message packets to a destination satellite delay compensation unit over the satellite channel and ceases transmission for a period known as "window" to await reception of acknowledgements. While this "window" procedure is efficient for network throughput, the multiple access and broadcast capability of the satellite channel is not utilized effectively because it is used as a preassigned, dedicated circuit.
On the other hand, Advanced Research Projects Agency Network (ARPANET) employs a satellite channel as a part of the network. To take advantage of the multiple access, broadcast capability of the satellite, each earth station examines the address of received messages and processes those addressed to it. Two alternative methods are employed to ensure transmission reliability. One is to control the timing of transmission so that no data collisions occur on the satellite link, and the second is to relay data as it is received from a terminal without delay to the satellite and retransmit a copy of the previous data if it encounters collision. However, this network employs an end-to-end protocol in which an acknowledgement is returned from the destination terminal over the satellite link in response to each transmission of a message packet, thus imposing limitations on high speed data communication.